Spinal fixation plates can be used for a variety of conditions, including for example, providing added strength and rigidity after fusion of adjacent vertebral bodies for securing vertebrae together where an intervening vertebral body has been removed and replaced. Generally, a spinal fixation plate is applied to the anterior side of affected vertebrae to span at least one affected disc space. For example, a spinal fixation plate may be applied to adjacent vertebral bodies where at least a portion of a disc has been removed and a spinal fusion spacer has been inserted.
Generally, a spinal plate may be attached to the anterior of two or more vertebral bodies for the purpose of immobilizing, stabilizing, and/or aligning those vertebrae. Additionally, such a plate may be used, for example, to supplement the function of an intervertebral spacer or artificial disc, to prevent an intervertebral spacer from being expelled from an intervertebral disc space and/or to act as a support for biocompatible bone graft material that is implanted in the disc space.
Orthopedic fixation devices such as spinal plates may be coupled to bone with fasteners inserted through openings in the plates. The fasteners may or may not be secured to the plate. It is known to secure such fasteners to a bone plate, for example, through the use of threads on the fastener and matching threads on the plate, though other means of securement are available. Such a screw-plate interface may decrease the incidence of loosening of the fixation assembly post-operatively. It is also known that a bushing may preferably be disposed in each plate hole to receive the fastener to permit polyaxial movement so that the fastener may be angulated at a surgeon-selected angle. While polyaxial movement of fasteners through set plate hole locations may increase attachment alternatives of the fasteners themselves, the plate holes remain fixed in relation to each other and to the longitudinal axis of the plate. Consequently, undesirable loads may be imposed on the plate fasteners as vertebral bodies subside after a spacer and/or bone graft material is implanted in the intervertebral disc space of adjacent vertebrae.
Further, screw blocking systems are generally provided in a bone plate to keep the fasteners from backing out of the plate. In the present invention, each opening in the plate preferably has a groove or recess for receiving a split ring, though any other suitable screw locking systems may be used in connection with the present invention.
Split rings may be pre-assembled to the bone plate. A split-ring can be sized to expand upon insertion of a bone screw into an opening in the bone plate. Once the head of the screw has passed through the split ring, the split ring can contract under its natural spring tension. When the ring relaxes to its unexpanded state, it prevents the bone screw from backing out of the plate by the engagement of an undersurface of the split-ring and an upwardly facing surface on the bone screw. U.S. Pat. No. 6,602,255, titled “BONE SCREW RETAINING SYSTEM” and issued on Aug. 5, 2003 and U.S. Pat. No. 6,261,291, titled “Orthopedic Implant Assembly” and issued on Jul. 17, 2001, both disclose devices used for securing bone screws to a bone plate and are incorporated herein by reference in their entirety as if fully set forth herein.
Generally, after implanting the spacer between a pair of vertebrae, there is a compression of the spacer between the adjacent vertebral bodies. This compression ensures a good engagement between the spacer and the endplates, increasing the chances that fusion will occur. Often, particularly in the period immediately following surgery, the spacer may subside slightly into the endplates. In the case of allograft spacers, the space between the vertebral endplates may decrease due to graft resorption.
Where a rigid fixation plate is used to connect vertebral bodies, this subsidence may tend to shift more of the spinal load to the plate than is desirable. Such load shifting can also occur due to inaccuracies in installing the plate to the vertebrae. In extreme circumstances, this load shifting can result in non-fusion or incomplete fusion between the adjacent vertebral bodies.
It is known in the art to provide a spinal plate which may be adjustable along the longitudinal axis between a plurality of positions. These plates may generally be described as incremental locking plates that are not infinitely adjustable. Such plates only allow for a first plate and a second plate to be assembled in a finite number of fixed positions with respect to one another by a surgeon or through natural subsidence after implantation. Moreover, many of the plates cannot be extended once locked in a fixed position, and this restricts flexibility during surgery and in revisions.
Accordingly, there exists a need for a fixation system having plates susceptible to infinite adjustment between a first and second assembled position. Such a system preferably includes plates that may freely subside in a first direction, while preventing movement of the plates in a second direction, as well as having the ability to unlock so that the plates can move in the second direction if desirable or necessary. Such a plate preferably provides the desired support to the vertebrae to be fused, and allows compression of the vertebrae with respect to at least a portion of the plate, thereby limiting the undesirable effects of load shielding by the plate due to graft subsidence caused by settling or normal forces experienced in the spinal column.